The present invention is related to charger circuits for battery cells. More particularly, the present invention is related to a protection circuit that clamps the charger circuit input when a no power source is detected.
Rechargeable lithium bases batteries, including Lithium-Ion batteries and Lithium-Polymer batteries, are used in portable applications such as cellular telephones. Lithium batteries are sensitive to excessive voltages. Without a suitable safety circuit overcharging may compromise the batteries reliability. A shunt regulator is often employed to regulate the charging voltage to the lithium battery.
An example shunt regulator charging system for a lithium battery is shown in FIG. 1. As shown in FIG. 1, the shunt regulator charging system includes a power source (PS), a shunt regulator (102), and a lithium battery (BATT). The power source (PS) includes a voltage source (VS) and a source resistance (RS). The shunt regulator (102) includes a NMOS transistor (MN), a PMOS transistor (MP), an amplifier (AMP), and three resistors (R1-R3).
In operation the power source provides a charging current (I) to the lithium battery through source resistance RS, PMOS transistor MP, and resistor R3. Resistor R3 converts the charging current (I) into a voltage (VSNS), which is used by other circuitry (not shown) to control the activation of transistor MP. PMOS transistor MP is activated during normal charging operations. Resistors R1 and R2 form a voltage divider that provide a feedback signal to amplifier AMP. Amplifier AMP compares the feedback signal to a reference voltage (VREF) and provides a control signal to transistor MN. Transistor MN, amplifier AMP, and resistors R1-R2 together operate as a shunt regulator that regulates the input voltage (VIN). The shunt regulator provides safe charging of lithium battery BATT by limiting the charging voltage (input voltage) similar to a zener diode.
A system and method is related to charger systems that require fault protection at an input terminal. A clamp circuit is activated when the power source is detected as disconnected from the system. The clamp circuit operates as a clamp that limits the input voltage by shunting current away from the input terminal. The clamp circuit operates as a standby protection circuit that may be part of shunt regulator and/or a crowbar system in a shunt regulator. The clamp circuit prevents an xe2x80x9cin-rushxe2x80x9d of current, due to a fault in the regulator, from creating a dangerous condition in a charging device (i.e., battery operated) when the power source is initially engaged. The clamp circuit is deactivated when the detected power supply voltage is above a battery voltage of the device.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of illustrative embodiments of the invention, and to the appended claims.